Automatic gain control



p -2 3 w. s. BARDEN 1,974,909-

' AUTOMATIC GAIN CONTROL Filed April 15. 1951 (Jill/Z4770 MEINS I 1 v Ia 1% it:

INVENTOTR mum 5. moan ATTORNEY BY w m Patented Sept. 25, 1934 1,974,909AUTOMATIC GAIN,ooN'm'oL William S. Barden, Stapleton, Staten Island, N.Y., assignor to Radio Corporation of America, a corporation of DelawareApplication April 15', 1931, Serial No. 530,267

8 Claims.

My present invention relates toamplifiers, and more particularly toanautomatic gain control arrangement for use, in connection with a radiofrequency amplifier circuit.

5 It is well known in the prior art to employ an accumulator device inthe grid circuit of a detector tube to secure automatic volume controlof a radio receiver. Generally, such an arrangement comprises a capacityin series between the cathode and grid of the detector tube, and aresistor connected in. shunt with thecapacity. The operation of such a.gain control arrangement may be explained by pointing out that signalenergy, applied to the input circuit of the detector tube, whose peakvoltage magnitude exceeds the detector grid bias value results in thecharging of the aforementioned series capacity due togrid current. Theeffective detector grid bias is, then, the sum of the fixed andaccumulated biases.

t It is, of course, obvious that the accumulated bias, in such a case,is. variable with signal field strength, and,'also, with percentagemodulation. The consequent changes in bias result in surges in the audiofrequency system, with the result that there simultaneously occurssurging of the reproducer diaphragm; Other forms of distortion andundesired effects may, also, result. For example, the; variation ofdetector grid" bias com- 30 prises. an obstacle to the desiredautomaticvolume control operation.

7 Now, I have discovered a method of, and devised novel means for,eliminating the aforementioned difiiculties in an automatic gain controlarrangement of the type described, the method-consistingin associating atwoel'e'etrode-space discharge system with the control electrode andcathode of the detector tube, and disposing the accumulator controlcircuit in the said two electrode system, with the result that theaccumulator control circuit is operated by the two electrode system; andthe detector tube has a fixed bias applied at all times to its controlelectrode, as is desired for optimum reception and'reproduction ofsignal energy;

Accordingly, it is one of the main objects of my present invention toprovide an automatic gain control arrangement for a radio receiver,thearrangement comprising a: two. electrode: space 5.0 discharge tube.disposed in the input, circuit of the receiver detector tube, and anaccumulator circuit connected between. the two electrodes of said space.discharge. tube whereby the detector stage of a radio receiver may beemployed for v55 signaltranslationaswell as automatic gain conpurposes.

trol of the radio frequency. amplifier, and yet the two functions ofgain control and transla.-- tion be independent. of each other.

Another important object of the present; invention is to provide in'aradio receiver includ ing a radio frequency amplifier and a: detectortube, a diode connected in shunt with the control electrode and cathodeof the detector" tube, and an accumulator circuit including a capacityconnected in series between the electrodes of the diode, a resistor inshunt with said capacity, and additional means for connecting a pointonthe input of the diode; to one, or more, points in the grid circuits ofthe radio frequency amplifier.

Otherobjects of the invention are to improve generally the efficiency ofautomatic gainv control arrangements of the present type, and toparticularly provide a gain control receiver which is-not only reliablein operation, but economically manufactured and assembled;

The novel features which I believe to be characteristic of my inventionare set forth. in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenconnection with the drawing in which I have indicated diagrammaticallyone circuit organization whereby my invention may be carried intoeffect.

Referring tothe accompanying drawing, there is shown, merely by way ofillustration, a radio receiver of the so-c-alled tuned radio frequencyamplifier type. The receiver in its usual form includes a groundedantenna system 1, which is coupled,- as at M,-to the input circuit of"the first radio frequency amplifier tube- 2, the said input circuitbeing tuned by a variable condenser 3.

The output of the tube 2is coupled, as at M1, to

the input circuitof a second radio frequency amplifier tube 4, theinputcircuit of this tube being tuned by a variable condenser 5. While twostages of radio frequency amplification have. only been shown, it is'tobe clearly understood that as many tuned stages as desired may bev em,-

ployed.

The. amplifi u put of the. last amplifier tube is impressed, through acoupling. M2, upon the input circuit of a. detector tube 6, the latterincluding the," usual series capacity 7 inv the grid circuit. thereof;and a grid leak a for detection 7 The. detected output is, impressed,throu h anaudio frequency transformer coupling M3, upon any well knowntype of utilization means. The latter may be one, or more, stages ofaudio frequency amplification followed by any type of reproducer wellknown to those skilled in the art, as, for example, a loud speaker ofthe electromagnetic, or electrodynamic type, or any other type ofreproducer of audio frequency impulses.

The anode circuit of each of the tubes 2, 4 and 6 includes a source ofanode potential l3, these sources being shown independent for the sakeof simplicity of description only, it being clearly understood thatother types of potential sources may be readily applied to the presentinvention, such sources being numerous and well known in present powersupply practice. Furthermore, the control electrodes of each of thetubes 2, 4 and 6 are shown biased by means of resistors R disposed inthe low potential, or ground, sides of the cathodes of these tubes.

In order to maintain the amplification of the amplifier stages,including the tubes 2 and 4, at a predetermined level, there is providedan automatic gain control arrangement. The latter comprises a diode 9having its anode connected to the grid circuit of the detector tube 6,and its cathode to the ground side of the cathode of the detector tube.In other words, an auxiliary space discharge system including an anodeand cathode is disposed in shunt to the control electrode and cathode ofthe usual detector tube. An accumulator circuit comprising a fixedcapacity 10 and a resistor 11, the latter being in shunt with thecapacity 10, is connected in series with the grounded leg of the diodecathode.

Aconductor 12 is connected between the high potential side of theaccumulator circuit and the grid circuits of the amplifier tubes 2 and4, the conductor being connected to the low potential sides of theaforementioned grid circuits in a manner well known to those skilled inthe art. A blocking capacity 13 is disposed between the anode potentialsource B and the gain control connection to each grid circuit in each ofthe amplifier stages.

The operation of the automatic gain control mechanism is believed to beclear from the above description. It is pointed out, in review, that theaccumulator circuit 10, 11 is operated by the diode 9, and the detectortube 6, by virtue of the present arrangement, has a fixed bias appliedto its control electrode, as is desired in an arrangement of this type.When signal energy above a predetermined value is applied to the inputcircuit of the detector stage, the internal impedance of the diode 9will be of such a magnitude that it will become conductive, or in otherwords break down and permit the flow of current therethrough, with theresult that the capacity 10 becomes charged.

The direct current potential accumulated across the circuit 10, 11 is,then, utilized to control the grid bias of the amplifier tubes 2 and 4.Obviously, when the signal energy applied to the input of the detectortube 6 exceeds the desired predetermined value, the direct currentpotential developed across the accumulator circuit will serve toincrease the bias on thegrids of the amplifier tubes. The amplificationwill, then, be decreased sufiiciently to restore the magnitude of theenergy applied to the detector input to the desired level. The reverseaction, of course, takes place whenever the signal energy applied to theinput circuit of the detector tube falls below the desired level. Inthis way, the gain of the radio frequency amplifier stages is maintaineduniform.

It will now be appreciated that there has been provided by means of thepresent invention a simple arrangement, employing an accumulator circuitin the detector input, for controlling the gain of a radio frequencyamplifier, the particular feature of the present arrangement residing inthe fact that the gain control arrangement does not interfere with thedetector action of the detector stage due to the use of an auxiliarydiode in the input circuit of the detector tube.

While the present receiver has been shown to be of the tuned radiofrequency type, it is to be understood that the invention is not limitedto such a receiver. However, for simplicity of description, the inputcircuits of the tubes 2, 4, and 6 are shown as having their tuningcondensers arranged for mechanical uni-control, as denoted by the dottedline 20, it being pointed out that the variable condenser 21 is employedfor tuning the input circuit of the detector stage.

While I have indicated and described one arrangement for carrying myinvention into eifect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

I. In combination, in a gain control system for a radio receiver, amulti tube radio frequency amplifier, a detector tube having an inputcircuit, a diode disposed between the input electrodes of the detectortube, a condensive accumulator'circuit connected in series between thediode electrodes and between the detector'input electrodes, and aconductive connection between the accumulator circuit and the gridcircuit of at least one tube in the radio frequency amplifier, saidcondensive circuit being connected in series with the detector inputcircuit.

2. A radio receiver comprising at least one stage of radio frequencyamplification including an electron discharge tube, a detector tubecoupled to the output circuit of the amplifier tube, a space dischargedevice including at least two electrodes connected in the input circuitof the detector tube, 'a capacity between the electrodes of the spacedischarge device, a resistor in shunt with said capacity and in serieswith the detector input circuit,'means independent of said device forkeeping the detector grid fixedly biased at all times, and a connectionbetween one side of said capacity and the grid circuit of said amplifiertube.

of radio frequency amplification including an electron discharge tube, adetector tube coupled to the output circuit of the amplifier tube, adiode connected in the input circuit of the detectortube and having itsanode connected to the detector pled to the output circuit of theamplifier tube,

3. A radio receiver comprising at least one stage 1 a diode connected inshunt with the input electrodes of the detector tube, a capacity inseries between the electrodes of the diode, a resistor in shunt withsaid capacity and in series with the detector input circuit, a detectorgrid bias means in the detector cathode circuit, a connection betweenone side of said capacity and the grid circuit of said amplifier tube,said diode maintaining the detector grid bias free from variation bypotential changes across said resistor.

5. In a high frequency receiving circuit, an amplifier, means connectedwith an electrode of the amplifier for controlling the transmissionefficiency of the amplifier, a detector having an input circuit, saidmeans including a condensive network connected between the inputelectrodes of the detector and in series with the detector inputcircuit, and a space discharge path con nected in shunt with saidnetwork for developing a potential across said network when signalsapplied to the amplifier input exceed a predetermined intensity, andmeans independent of said path and network for maintaining a fixed biason the detector grid throughout development of said potential.

6. In a high frequency receiving circuit, an amplifier, means connectedwith the input electrode of the amplifier for controlling thetransmission emciency of the amplifier, a detector having an inputcircuit, said means including a condenserresistance network connectedbetween the input electrodes of the detector and in series with thedetector input circuit, and a space discharge path connected in shuntwith said network for developing a potential across said network whensignals applied to the amplifier input exceed a predetermined intensity,and means connected between the detector grid and cathode, andindependent of said path, for maintaining a substantially fixed bias onthe detector grid at all times.

'7. An automatic gain control system for a radio frequency amplifiercomprising a rectifier circuit including an electron discharge tube, acontrol network in series between the input electrodes of the tube, thelatter electrodes being coupled to the amplifier output, a diode inshunt with the network and connected between said input electrodes, anda conductive connection between one side of the network and an amplifierinput electrode.

8. An improvement in an automatic volume control arrangement for highfrequency amplifiers of the type comprising a detector having its inputcircuit coupled to said amplifier, means for biasing the detector gridbeing disposed between the grid and cathode of the detector, acondenserresistor ccumulator network being connected between thedetector input electrodes in series with said detector input circuit,and a gain control electrode of said amplifier being conductivelyconnected to one side of said network, said improve ment including aspace discharge device connected across the said input electrodes, oneelectrode of said device being connected to the other side of V saidnetwork, and a capacity disposed between the device and the grid of saiddetector, said device and capacity preventing potential changes acrosssaid network from affecting the detector grid bias.

WILLIAM S. BARDEN.

